Electrically programmable semiconductor fuses, or electrical fuses in short as it is referred to in this invention, have been used in semiconductor circuits to provide alterations in the functionality of the circuitry. Typical examples of applications of electrical fuses include: providing redundancy to enable repairs of imperfect chips, storage of secure and permanent information, selection of a particular configuration for chip operation, tuning analogue circuit components, optimizing overall circuit performance, and/or replacing defective circuit elements with redundant circuit elements.
Electrical fuses are programmed by the physical alteration of the structure of the electrical fuses. The most commonly used structure of electrical fuses employs a vertical stack comprising a semiconducting material and a conducting material. While the most common material for the vertical stack is polysilicon and silicide, other semiconducting materials and other conducting materials may be utilized if similar electromigration properties can be found in the combined stack of the two materials. In general, the stack comprises a layer of semiconductor material and a layer of a metal semiconductor alloy, which may be a silicide. This stack is patterned such that a narrow and long piece of material, called “fuselink” or “fuse neck,” is adjoined by two large plates, called “cathode” and “anode” respectively, depending on the polarity of electrical bias applied to the electrical fuse during programming. Electrical current of relatively high density flows through the fuselink when a sufficiently high voltage bias is applied across the cathode and the anode. The programming current may be high enough to cause the electrical fuses to rupture by a sudden increase in temperature in the physical structure of the electrical fuses. This type of programming is commonly referred to as “rupture mode programming.” Alternatively, the level of the programming current may be moderated to cause a controlled electromigration of the material inside the electrical fuse structure. This alternative mode of programming is commonly referred to as “electromigration mode programming.” Both programming methods raise the resistance of the programmed fuse compared to that of intact fuses.
By measuring the resistance of electrical fuses, it can be determined whether the electrical fuse has been programmed or intact. While it may not be necessary to measure the exact value of the fuse resistance to determine the state of the fuse, it is generally necessary to determine whether the fuse resistance has been raised by a significant amount above the detection limit of the sensing circuitry. Typically, this is done by setting the resistance for a reference resistor at a value about 3˜10 times that of an intact electrical fuse and comparing the resistance of the fuse with that of the reference resistor. A difference between the resistance of the reference resistor and the resistance of an intact fuse is often necessary to insure margin in the functionality of the sensing circuitry under adverse operating conditions of the chip.
Reliable programming of electrical fuses in an electromigration mode requires a minimum level of divergence of electrical current density at the cathode to induce electromigration of a metal semiconductor alloy at the cathode. While the divergence of electrical current is proportional to the magnitude of the electrical current through the fuselink, the size of a programming transistor, and correspondingly, the area occupied by the programming transistor, are proportional to the magnitude of the electrical current to be supplied to the fuselink during programming. However, the divergence of electrical current is also a function of geometry of the electrical fuse structure. In principle, the paths of the electrical current for programming an electrical fuse may be engineered to induce a higher level of divergence of electrical current density by manipulating the geometry of the electrical fuse structure.
Therefore, an electrical fuse structure that produces a high level of divergence of electrical current at the cathode, and methods of manufacturing the same are desired.